This invention relates to printed wiring boards for recording or displaying information comprising a substrate, functional elements placed on the substrate, and strip wires connected to the functional elements. In particular, the invention is directed to an improvement for correcting resistance unbalance of strip wires due to the different lengths of strip wires employed.
There is a printed wiring board which comprises an insulated substrate, functional elements placed on the substrate, and strip wires connected to the functional elements. For example, the invention can be utilized with a thermal printing head which comprises an insulated substrate, a plurality of thermal resistive elements aligned on the substrate, and a plurality of strip wires formed on the substrate and connected to the thermal resistive elements. Moreover, the invention can be utilized with a liquid crystal display apparatus which comprises two substrates, having the liquid crystal sandwiched between the two substrates, a plurality of electrodes formed on the substrate, and a plurality of strip wires formed on the substrates and connected to the electrodes. In such an apparatus, a functional element consists of electrodes facing one another and a liquid crystal positioned between the electrodes.
In either of these above described printed wiring board embodiments, the length of the strip wires connected to the functional elements on the substrate depends upon the positions of the elements vis-a-vis the electric source. Consequently, strip wires having various lengths are the inevitable result. When the area of the substrate upon which the functional elements are placed is small, the differential length of the longest strip wire and shortest strip wire is small. However, because printed wiring boards having a larger area of formed elements are more useful in a printing or displaying apparatus, such boards have been more frequently used. Therefore, the differential length of the longest wire and the shortest strip wire can not be ignored.
The larger the differential length of the longest and shortest strip wires or the larger the differential width of the strip wires, an increase in the differential resistance will occur. As a result, the voltage or electric current supplied to each functional element will not be uniform. In the case of thermal resistive printing elements, undesirable copies will be obtained which have a remarkable contrast in printing density. That is, the copies will have light and dark printed portions. In the case of liquid crystal display elements the displayed image will have light and dark density portions.
For example, in the thermal head circuit shown in FIG. 1, thermal resistive elements are formed on a substrate. These elements Rij are aligned on the substrate, wherein one end of these elements are connected to respective positional selective terminals Bi by strip wires. The terminals are connected via semiconductor diodes Dij to prevent leakage of current. As shown in FIG. 1, there are seven terminal groups (B1, B2 . . . ) each consisting of five thermal resistive elements (R11, R12 . . . ) such that i=1, 2, . . . , 7 and j=1, 2, . . . , 5. The other end of thermal resistive elements Rij are connected to signal terminals Cj. In operating an element, the selected terminals Bi is grounded and the activating recording signal is supplied to one or more of the five thermal resistive elements Ri1 . . . Ri5 via the appropriate signal terminals C1 . . . C5 in parallel. For example, if selected terminal B1 is grounded and negative voltage is supplied into signal terminals C2, C4, thermal resistive elements R12 and R14 of group R11 . . . R15 will be activated to print an image.
In the thermal printing head shown in FIG. 1, the length of the strip wires between the elements and their respective signal terminals are different. The strip wire between thermal resistive element R11 and signal terminal C1 is the shortest, while the strip wire between thermal resistive element R75 and signal terminal C5 is the longest. The difference in length of these strip wires is L1+L2. The longer L1+L2 becomes, the more severity in density contrast will result. Length L1 defines a boarder of the recordable area (i.e. the area in which printing takes place). Therefore, the longer the recordable area, the larger the differential resistance of the longest and shortest wire, thereby resulting in a more severe density contrast.